feat: FastAPI /analyze — MERT boundaries + KS key detection + guitar info

feat: smoother — majority-vote window smoothing + consecutive segment merger

feat: key detector — Krumhansl-Schmuckler 24-key Pearson correlation

feat: MERT boundary detector — cosine novelty curve + Gaussian smoothing

feat: MERT encoder — HuggingFace m-a-p/MERT-v1-95M, 768-dim embeddings per chunk

feat: chromagram extractor — chroma CQT mean, L1-normalized

feat: audio loader — load bytes, resample 22050 Hz mono, chunk with overlap

feat: music theory utils — scale notes, pentatonic, fretboard positions

chore: backend project scaffold with dependencies